The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then neutralized with an acid.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives (usually at the coating site) to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking. It is then typically covered with any of a variety of different topcoat systems (e.g. basecoat/clearcoat).
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.
The pigment dispersant is a very important part of an electrocoat primer composition. The dispersion process involves the separation of the primary pigment particles from their agglomerates or aggregates, the displacement of occluded air and absorbed water, and the wetting and coating of the pigment surfaces with the dispersion resin. Ideally, each primary particle, having been mechanically separated during dispersion, is also stabilized against flocculation. If the pigment particles are not properly dispersed and stabilized in the paint, the advantages built into the pigment by the manufacturer may be lost. For instance, the pigment may settle in the electrodeposition bath which can result in loss of corrosion protection of the substrate.
In addition, surface appearance, operating characteristics and so forth may be adversely impacted by inadequate pigment dispersion.
An improved pigment dispersant can result in increased production capacity, and lower volatile organic content (VOC) in the electrodeposition bath. Current commercial pigment dispersants used in cathodic electrocoat processes typically are polyepoxide resins containing either onium salts or amine salts. Using the pigment dispersants known in the prior art, the typical pigment to binder ratio that can be obtained is about 3:1. These pigment dispersants also require the use of solvents which raise the VOC of the electrocoat bath. Current commercial pigment dispersants contain at least 30 to 40% solvent. Furthermore, pigment dispersants of the prior art are not known to increase film build.
What is needed is a pigment dispersant which will allow improved processability, a lower VOC in the electrocoat bath, and increase the film build.
Another important part of electrocoat compositions are additives. Numerous different types of additives can be added. For instance, additives are sometimes used to increase film build. High film build is important to maintain equivalent corrosion resistance without the use of a primer surfacer over the electrocoat. Flow additives are also used to improve film smoothness.
What is needed is an additive which will both increase film build and give a film with excellent smoothness.